* He was Swiss: Paul Klee was born and lived primarily in Switzerland, where German is the primary language.
* His artistic travels: Though he traveled to Germany, Italy, and Tunisia, there's no record of him spending significant time in English-speaking countries.
* His writings: Klee's extensive writings and journals are almost entirely in German.
While it's possible he picked up a few English phrases during his travels, it's highly unlikely he achieved fluency.